In operation, such a component is subjected to a certain heating effect which depends, on the one hand, on the load current and, on the other hand, on the ability of the surrounding medium to dissipate the generated heat. On the basis of the maximum permissible temperature it is possible to optimize the various electrical operating parameters of the circuit arrangement. There is, however, a risk that the component may be destroyed by overheating due to unfavorable circumstances such as, for example, an accidental short-circuiting of the load.
In integrated circuitry, in which the component to be protected against destructive overheating forms part of a semiconductor wafer along with other circuit elements, it has already been suggested to use the temperature dependence of the voltage drop across a p/n junction for the detection of incipient overheating. German published specification No. 2,718,762, for example, describes the provision of an ancillary transistor in the vicinity of a principal transistor to be protected, the two transistors being formed jointly in the same substrate; the collector current of the ancillary transistor is utilized to trip a control circuit for limiting the conduction of the principal transistor as soon as the permissible operating temperature is exceeded.
In the case of a discrete transistor formed by a semiconductor chip it has heretofore not been possible, on account of the different technique generally employed in making such a chip, to provide a similarly effective temperature sensor. The manufacture of such discrete components requires that a large portion of the semiconductor substrate, specifically a region bounded by one of the major chip surfaces, be overlain by one of the main electrodes of the component, generally the one connected by way of the load to a live terminal of the power supply whose other terminal is grounded. The main electrode referred to is the collector of a bipolar transistor (or the common collector of the pilot and final transistors of a Darlington pair) or the drain of an IGFET.
Thus, conventional protective circuitry for a discrete transistor does not directly utilize the phenomenon of a temperature dependence of a voltage drop across a p/n junction. British Pat. No. 1,476,089, for example, describes a temperature-sensing resistor thermally coupled to the casing of the component to be protected. German published specification No. 2,656,466 discloses a temperature sensor in the form of a bridge circuit including the base/emitter junction of the protected transistor in one of its arms; when the junction temperature surpasses a predetermined value, its resistance diminishes and unbalances the bridge to actuate a controller for cutting off the transistor.
Devices using separate heat-sensitive resistors as overload detectors respond only with a certain delay to an excessive rise in temperature and therefore do not provide reliable safeguards against short circuits. Such devices, moreover, require special mechanical connections between the semiconductor casing and the external resistor to assure a reasonably good heat transfer. Protective bridge circuits of the type referred to have the drawback that the base/emitter voltage V.sub.BE of the transistor to be protected is not invariably the proper criterion for overload detection since that voltage is affected not only by temperature but also by other parameters such as the collector and emitter currents of that transistor. A device of the latter nature, therefore, is unsuitable where these currents may vary widely in normal operation.